Canadian Pat. No. 1,196,345 (1985) describes a process for the preparation of CF.sub.3 CHXY (X=H, F; Y=H, F, Cl, Br, I) by addition of HF to the corresponding ethylene in the presence of chromium oxyfluoride at 20.degree.-200.degree. C., especially 60.degree.-180.degree. C.
U.S. Pat. No. 3,755,477 describes a process for producing fluorinated aliphatic hydrocarbons which comprises fluorinating a halogenated aliphatic hydrocarbon, including tetrachloroethylene and chlorotrifluoroethylene, by reaction in the gas phase with HF in the presence of a steam-treated and calcined chromium oxide catalyst prepared by a multi-step process. Example 23, column 5, shows tetrachloroethylene as a raw material with formation of CF.sub.3 CHCl.sub.2 (20%), CF.sub.3 CHClF (20%), CF.sub.3 CHF.sub.2 (30%), and CF.sub.3 CClF.sub.2 (20%) at 10/1 HF/C.sub.2 Cl.sub.4 mol ratio, 5.4 seconds contact time and 360.degree. C. reaction temperature. Example 24, column 5, shows chlorotrifluoroethylene as a raw material with formation of CF.sub.2 .dbd.CF.sub.2 (20%) and CF.sub.3 CHClF (13%) at 1.8/1 HF/C.sub.2 ClF.sub.3 mol ratio, 4 seconds contact time and 320.degree. C. reaction temperature. In these examples, less desirable pentafluorinated products are obtained in a greater amount than the desired tri- and tetrafluoro products.
U.S. Pat. No. 3,258,500 describes a process for the catalytic vapor phase reaction of HF with halohydrocarbons, including tetrachloroethylene and chlorotrifluoroethylene, employing a catalyst that consists essentially of a heat-activated anhydrous chromium (III) oxide which may be supported on alumina. This catalyst is highly active. Example 17, column 14 showing fluorination of tetrachloroethylene with this catalyst, like that of the above '477 patent, produces large quantities of the less desirable highly fluorinated pentafluoroethane. At 400.degree. C. the product distribution is 35.0% pentafluoroethane, 9.2% 1-chloro-1,2,2,2-tetrafluoroethane, and 3.5% 1,1-dichloro-2,2,2-trifluoroethane. At 300.degree. C. the product distribution is 38.3% 1-chloro-1,2,2,2-tetrafluoroethane, 25.4% pentafluoroethane, and 16.0% 1,1-dichloro-2,2,2-trifluoroethane. Example 20, column 15, shows that chlorotrifluoroethylene yields CF.sub.3 CHF.sub.2 as the major product at 400.degree. C.
GB No. 1,000,485 describes a process for the preparation of organic fluorinated compounds by fluorination of halo-olefins in the gaseous phase and at a temperature preferably withintthe range of 200.degree. C. to 400.degree. C. The catalyst consists essentially of partially fluorinated alumina impregnated with one or more polyvalent metal halides. The polyvalent metal may be chromium, cobalt, nickel or manganese. The total content of polyvalent metal halide, expressed as oxide, is not more than 15% by weight of the partially fluorinated (70-80%) alumina expressed as alumina. Example 4 (Table 4) shows that reaction of tetrachloroethylene with HF over such catalyst yields CF.sub.3 CHCl.sub.2 as the major product at 220.degree.-290.degree. C. In addition, the patent states that, if fluorination of the catalyst is excessive, the activity of the catalyst is impaired (page 3, column 2, lines 85-87).
The references do not disclose how to favor the production of 1,1,1-trifluorodichloroethane over 1,1,1,2-tetrafluorochloroethane while maximizing the production of these two compounds taken together over the production of more highly fluorinated products.
The process of the instant invention achieves the desired high degree of selectivity by minimizing the formation of the pentafluoroethane through catalyst selection and control of the reaction variables, as discussed below and illustrated in the Examples.